/Engine/Plugins/FX/Niagara/Source/Niagara/Public/NiagaraCommon.h
Simple struct intended to hide the details of passing a reference to compiled data. In particular for editor builds it will actually make a copy of the data to help try to avoid the many edge cases of recompiling/editing, while regular builds can reap the wins of just referencing the data and saving memory.
template<typename T> struct FNiagaraCompiledDataReference